TsgcWSPClient_MQTT › Events › OnMQTTPublish

OnMQTTPublish Event

Fires when the broker delivers an incoming PUBLISH message; exposes Topic, Text and MQTT 5 PublishProperties.

Syntax

property OnMQTTPublish: TsgcWSMQTTPublishEvent;
// TsgcWSMQTTPublishEvent = procedure(Connection: TsgcWSConnection; aTopic, aText: String; PublishProperties: TsgcWSMQTTPUBLISHProperties) of object

Default Value

—

Remarks

Main delivery event: raised every time the broker forwards a PUBLISH to this client for a topic matching one of its subscriptions. Regardless of the publish QoS (0, 1 or 2) the event fires after the transport-level acknowledgement sequence is complete: QoS 0 is fire-and-forget, QoS 1 after PUBACK, QoS 2 after the PUBREC/PUBREL/PUBCOMP exchange. Parameters:

• aTopic — topic name the broker matched.
• aText — payload as a string. For binary payloads use OnMQTTPublishEx instead so you can read the raw bytes.
• PublishProperties (MQTT 5) — PayloadFormat, MessageExpiryInterval, TopicAlias, ResponseTopic, CorrelationData, UserProperties, SubscriptionIdentifier and ContentType.

Example

procedure TForm1.MQTTPublish(Connection: TsgcWSConnection;
  aTopic, aText: String;
  PublishProperties: TsgcWSMQTTPUBLISHProperties);
begin
  Memo1.Lines.Add(Format('[%s] %s', [aTopic, aText]));

  // MQTT 5: honour a ResponseTopic if the sender set one
  if PublishProperties.ResponseTopic <> '' then
    MQTT.Publish(PublishProperties.ResponseTopic, 'ack', mtqsAtMostOnce);
end;

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